1. Field of the Invention
The present invention relates to a method for forming an electret containing positive ions.
2. Description of Related Art
Generally, the manufacture of an electret is performed by injecting electric charges into an insulation layer with a corona electrical discharge. However since, with an electret that has been manufactured by this method, there are the problems that the electric charge density in the insulation layer is low, and also that the electric charge level may decrease due to long-term use, accordingly a construction for an electret is desired that can provide a higher electric charge density and also a longer life.
In Japanese Laid-Open Patent Publication 2010-068643, a construction is disclosed for manufacturing a planar comb-tooth electrode incorporating an electret, in which so-called BT treatment (Bias-Temperature treatment) is performed by applying a voltage of around 500 V to around 1000 V to an alkali glass while heating it to a temperature that is less than the glass transition point, so that alkali ions within this alkali glass are shifted and a region that is depleted in alkali ions is created.
In recent years, by employing the MEMS technique to manufacture comb-tooth electrodes in a three-dimensional construction, a compact electricity generation element has been developed in which the electrostatic capacity between the opposing electrodes has been increased. For example, mutually opposed three-dimensional comb-tooth electrodes of this type are used in the electricity generation element disclosed in Japanese Laid-Open Patent Publication 2010-011547, and a silicon oxide layer is used as an insulation layer in which an electret is formed, with the electric charge of the electret being established upon this insulation layer by a corona electrical discharge.